It has been reported that when a copper phthalocyanine layer (CuPC) layer is provided as a hole injection layer in low molecular weight organic EL (hereinafter abbreviated as OLED) devices, there can be realized improvements in initial characteristics such as a lowering of drive voltage and a luminous efficiency and also in life characteristics (Non-patent Document 1: Applied Physics Letters, United States of America, 1996, Vol. 69, pp. 2160-2162).
On the other hand, with respect to organic EL devices using polymer light-emitting materials (hereinafter referred to as PLED), it has been reported that similar effects as the OLED device can be obtained when using, as a hole transport layer, a thin film of a polyaniline material (Patent Document 1: JP-A 3-273087, and Non-patent Document 2: Nature, United Kingdom, 1992, Vol. 357, pp. 477-479) or a polythiophene material (Non-patent Literature 3: Applied Physics Letters, United States of America, 1998, Vol. 72, pp. 2660-2662).
In recent years, there has been found a charge-transporting varnish which makes use of a highly-soluble low molecular weight oligoaniline material or oligothiophene material and is made of a homogeneous solution completely dissolving the material in an organic solvent. A report has been made that when a hole injection layer obtained from the varnish is inserted into an organic EL device, a flattening effect of an underlying substrate and excellent EL device characteristics are obtained (Patent Document 2: JP-A 2002-151272 and Patent Document 3: WO 2005/043962 Pamphlet).
Such a low molecular weight oligomer compound is in its own low in viscosity, for which if an ordinary organic solvent is used, a process margin in film-forming operations becomes narrow. Accordingly, when using various coating systems such as spin coating, inkjet coating, spray coating and the like and a variety of baking conditions, a difficulty is involved in film formation ensuring high uniformity. Nevertheless, when using different types of additive solvents, adjustment of viscosity and controls of boiling point and vapor pressure become possible, thus enabling film-formed surfaces to have high uniformity correspondingly to various types of coating systems (Patent Document 4: WO 2004/043117 Pamphlet and Patent Document 5: WO 2005/107335 Pamphlet).
The reason why no precipitation of solid matters occurs after addition of various types of solvents as set out above thereby ensuring solution uniformity is based on the high solubility and non-aggregation property of such a low molecular weight oligomer compound. In this sense, the dissolution characteristic of a charge-transporting material to be coated is very important. On the other hand, it has been reported that an oligoaniline compound is oxidized beforehand (preliminary oxidation) for use partly as a quinoid structure, enabling the shortage of a baking time (Patent Document 6: WO 2004/105446 Pamphlet). This technique is highly effective in film formation on a film substrate that would need low temperature baking at 160° C. or below.
However, this preliminarily oxidized oligoaniline compound is low in solubility and high in aggregation property, so that the varnish obtained by use of the compound has, in some case, presented problems on filterability, and uniformity. Polyaniline and oligoaniline compounds generally exhibit the highest solubility in a reduced state (leuco-emeraldin) and the highest conductivity in a semi-oxidized state (emeraldin). On the other hand, when oxidation proceeds excessively, a perniglaniline state results wherein quinoid structures are arranged in sequence, with the result that the conductivity lowers and solubility further lowers.
The preliminarily oxidized polyaniline and oligoaniline compounds are, in most cases, in a mixed state of perniglaniline and emeraldin. Owing to the partly existing perglaniline state, solubility may often lower, aggregation may occur in a solution, and defectives or irregularities may occur upon formation as a film. Accordingly, where preliminarily oxidized polyaniline and oligoaniline compounds are used for the purposes of shortening a baking time, lowering a baking temperature and the like, it is necessary to create an oxidized state wherein a highly dispersed semi-oxidized state is formed without formation of a sequence of quinoid structures. This formation is usually difficult and thus, there has been a demand for solving the problem.    Non-patent Document 1: Applied Physics Letters, United States of America, 1996, Vol. 69, pp. 2160-2162    Non-patent Document 2: Nature, United Kingdom, 1992, Vol. 357, pp. 477-479    Non-patent Document 3: Applied Physics Letters, United States of America, 1998, Vol. 72, pp. 2660-2662    Patent Document 1: JP-A 3-273087    Patent Document 2: JP-A 2002-151272    Patent Document 3: WO 2005/043962 Pamphlet    Patent Document 4: WO 2004/043117 Pamphlet    Patent Document 5: WO 2005/107335 Pamphlet    Patent Document 6: WO 2004/105446 Pamphlet